Field of the Invention
The present invention relates to a work vehicle and a work vehicle control method.
Background Information
A work vehicle, such as a wheel loader, which is provided with a power transmission device having a torque converter and a multi-stage transmission (the power transmission device is referred to hereinbelow as a “torque converter-type transmission”) has become publicly known. Recently however, hydraulic-mechanical transmissions (HMT) have become known as power transmission devices in place of torque converter-type transmissions. As disclosed in Japanese Laid-Open Patent Publication No. 2006-329244, an HMT has a gear mechanism and a motor connected to rotation elements of the gear mechanism, and a portion of the driving power from the engine is converted to hydraulic pressure and transmitted to the travel device, and the remaining portion of the driving power is mechanically transmitted to the travelling apparatus.
The HMT is provided, for example, with a planetary gear mechanism and a hydraulic motor to allow continuous speed variation. The first element among the three elements of a sun gear, a carrier, and a ring gear of the planetary gear mechanism is coupled to an input shaft, and the second element is coupled to an output shaft. The third element is coupled to the hydraulic motor. The hydraulic motor functions as either a motor or a pump in accordance with the travel state of the work vehicle. The HMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the hydraulic motor.
An electric-mechanical transmission (EMT) has been proposed as a technique similar to the HMT. An electric motor is used in the EMT in place of the hydraulic motor in the HMT. The electric motor functions as either a motor or a generator in response to the travel state of the work vehicle. Similar to the HMT, the EMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the electric motor.